am: format is in $patch_format, not parse_patch

The error message given when the patch format was not recognized was
wrong, since the variable checked was $parse_patch rather than
$patch_format. Fix by checking the non-emptyness of the correct
variable.
Signed-off-by: NGiuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -268,7 +268,8 @@ split_patches () {
 		msgnum=
 		;;
 	*)
-		if test -n "$parse_patch" ; then
+		if test -n "$patch_format"
+		then
 			clean_abort "Patch format $patch_format is not supported."
 		else
 			clean_abort "Patch format detection failed."